This　study　explores　the　startup　of　innovative　Partial　denitrification/Anammox　(PD/A)　process　using　long-term　stored　sludge　(＞2　years　at　4　C).　Results　indicate　a　swift　recovery　performance,　characterized　by　a　progressive　increase　in　the　activity　of　functional　microorganisms　with　improved　nitrogen　volumetric　loading　rate　during　operation.　Stable　nitrogen　removal　efficiency　of　99.6　%　was　attained　at　14.2　C　under　influent　nitrate　and　ammonium　of　120　and　100　mg/L,　respectively.　A　distinctive　transformation　was　observed　as　the　initially　black　seeding　sludge　transitioned　to　brownish-red,　accompanied　by　rapid　sludge　granulation　with　size　notably　increased　from　263.1　mu　m　(day　4)　to　1255.0　mu　m　(day　128),　significantly　contributing　to　the　rapid　PD/A　performance　recovery.　Microbial　community　analysis　revealed　substantial　increases　in　functional　bacteria,　Thauera　(0.09　%-10.4　%)　and　Candidatus　Brocadia　(0.003　%-1.98　%),　coinciding　with　enhanced　nitrogen　removal　performance.　Overall,　this　study　underscores　the　viability　of　long-term　stored　PD/A　sludge　as　a　seed　for　rapid　reactor　startup,　offering　useful　technical　support　to　advance　practical　PD/A　process　implementation.